1. Field of the Invention
The present invention relates in general to compositions and microlithographic processes that allow use of extremely thin photoresist layers to form microelectronic structures.
2. Description of the Prior Art
The advances of microelectronic manufacture are reflected by the density and dimensions of semiconductor structures created by the microphotolithographic process. The demand for high density and small critical dimension (CD) has been constantly pushing photolithography technology to its limits. To keep pace with the semiconductor industry roadmap, next-generation patterning materials and innovative lithographic processes will be needed to work in unison for high-resolution lithography. As critical feature sizes keep shrinking to 32 nm and beyond, and while the aspect ratios of printed lines have certain limits to avoid possible line collapse, the use of a thin photoresist has been widely accepted to give better resolution and a large depth of focus (DOF). Rayleigh's law can be used to define the pattern resolution and depth of focus (DOF):Resolution=k1λ/NA; andDOF=k2λ/NA2,where λ is the irradiation wavelength, NA is the numerical aperture of the exposure tool, and k1 and k2 are constants for a given process. Rayleigh's theory indicates that an exposure tool with short wavelength and large numerical aperture will produce better pattern resolution. This principle is why the microelectronics industry has been progressively moving toward short exposure wavelengths. However, Rayleigh's law also shows that enhancing resolution causes the DOF to decrease. The use of a thin photoresist decreases the value of k1 and increases the value of k2, which results in better resolution and a large DOF. However, the reduced photoresist thickness cannot offer sufficient etch resistance to transfer the pattern into the substrate, especially for 193-nm ArF photolithography. Due to the transparency requirement, aromatic structures cannot be put into ArF resists, so most ArF resists etch even faster than previous photoresists. To solve this conflict between the need for ever-thinner photoresists for better resolution and the need for a sufficient etch budget for pattern transfer, only a few process and material solutions currently exist.
In one approach, silicon- or metal-containing hardmasks have been introduced into a multilayer etch stack to help provide a complete pattern transfer. Reactive ion etching (RIE) is typically used to open the hardmask layer under the photoresist. Clearly, the hardmask-to-photoresist etch selectivity determines how thin the photoresist can be. Unfortunately, almost all currently-available photoresists still etch relatively rapidly under common hardmask plasma etch chemistries. Thus, the photoresist still must be substantially thick for high-resolution lithography.
Another solution is to use a developer-soluble underlayer material to eliminate the otherwise required etch step. Isotropically developable and photosensitive bottom anti-reflective coatings have been described. However, undercutting is very difficult to control in an isotropically developable bottom anti-reflective coating. For a photosensitive, anisotropically developable bottom anti-reflective coating, a major concern is bottom anti-reflective coating clearance and CD uniformity when it is coated on substrate topography. On the other hand, if they are coated on top of a spin-on carbon (SOC) planarization layer, those organic bottom anti-reflective coating materials are not effective as hardmasks.
More recently, multiple exposure technology for the next printing node has become the only viable option until exposure wavelengths shorter than 193 nm, such as 13.5 nm, are available. Many process schemes for multiple exposure technology have been investigated and reported. Some schemes utilized a bright field mask where only small portions of the photoresist, such as lines, are protected from the exposure, while the remaining portion of the resist is exposed. The photoresist is then contacted with developer to remove the exposed portions of the resist, thereby leaving only the unexposed portion of the photoresist (i.e., the lines) remaining above the hardmask layer. The pattern is transferred to the hardmask by etching away the hardmask layer except for those areas underneath the unexposed portions of the photoresist. The process is repeated until the desired pattern is achieved. In a dark field exposure process, a large portion of the photoresist is protected from exposure, while only the small portions of the photoresist are exposed and removed after development. As with bright field, the pattern must then be transferred to the hardmask using an etching process.
Likewise, ArF immersion lithography, together with double patterning, offers a more realistic solution for 32-nm and 22-nm half-pitch node fabrications. However, the current litho-etch-litho-etch (LELE) process is very expensive, even in comparison to EUV lithography.
Thus, many existing processes still require a dry-etch step to transfer the patterns to the underlying layer. The dry-etch step complicates the process and increases the cost and time involved. Accordingly, there is a need in the art for protective materials that can be used as a hardmask layer and over planarization layers or in multiple exposure processes that eliminate the need for the etching step and permit the use of extremely thin photoresist layers for increased DOF and CD control. There is also a need for hardmask layers that can be used with ArF immersion lithography, together with double patterning.